I have a feeling many will believe this answer is obvious; I am reaching the conclusion that the following will probably get the Raspberry Pi firmware update done for Ubuntu 22.04. Even moreso when I read Raspberry Pi firmware is in the vfat boot partition for both the RasPI OS and Ubuntu recognizing it has a different mountpoint in the respective OS.
apt update
apt upgrade
I know rpi-update is "bleeding edge" and generally not what I want. It is only intended for the RasPI OS with warnings about compatibility with other OSes another reason not to use it.
The only askUbuntu content was about trying to get Ubuntu releases for RasPI2/3 to run on RasPI4; should not be an issue for Ubuntu 22.04 that actually supports the RasPI 4, if as I now suspect upgrade alters the boot partition as required for each change.
If it matters the Uboot loader is compatible with Ubuntu 22.04 as I have booted it from a flash drive attached to a USB port. I checked several months ago and my PIs had the current uboot eeprom then.
Thanks